Rename images to sort by entropy (PIL needed)

gistfile1.py

import Image
import os
from math import log

def entropy(img):
  histogram = img.histogram()
  log2 = lambda x:log(x)/log(2)
  total = len(histogram)
  counts = {}
  for item in histogram:
    counts.setdefault(item,0)
    counts[item]+=1
  ent = 0
  for i in counts:
    p = float(counts[i])/total
    ent-=p*log2(p)
  return -ent*log2(1/ent)

for file in os.listdir('.'):
  os.rename(file, str(entropy(Image.open(file))) + '-' + file)